The use of discrete light emitting diode arrays for graphic displays is well known. Light emitting diodes will be described herein as LEDs. An individual LED chip is called herein a "die", and a plurality of LED chips are called "dice". Each picture element of a display is called a pixel, and a pixel may include LEDs of several colors which are separately or concurrently energized. For example, a single pixel may include red and green LEDs, and it is anticipated that blue LEDs are or soon will be available to complete the combination of red, blue and green LEDs for full color displays.
The method of placing a LED chip or die onto a substrate is called "die bonding".
It is apparent that methods for die bonding LED dice may be used to position other chips.
To create a high resolution picture, the pixels must be closely spaced, and the individual LED dice must be even more closely spaced. In a monochrome picture, each pixel would have only one LED die.
The size of the dice would be expected to limit the resolution, and so it does. As expected, a larger die is more difficult to place on a short center-to-center spacing than a smaller die. However, the LEDs cannot presently be positioned without spacing therebetween, for existing positioning or die bonding tools, or collets, are larger than the LEDs, and mechanical interference between the tool and the adjacent LED dice limits the array density and the picture resolution. Clearances between LED dice were necessary to allow for the size of the positioning or die bonding tool which holds the die during the positioning of the die on a surface or substrate.
LEDs are placed by die bonding machines. The machine used by the inventor was manufactured by Foton Production Automation Inc. of Chandler, Ariz. It is a model 8030 machine, serial number A01403063 with a vacuum tool or collet for the die.
Die bonding machines are well known in the art, and are available with varying degrees of automation. In a completely manual die bonder, the operator guides the bonding tool or collet to the LED die and picks it up by means of a vacuum. The operator then positions the die at the desired location on the substrate and releases the vacuum to leave the LED die in place. In a semi-automatic die bonder the operator still performs the delicate pick-up operation, but the die is automatically bonded to a predetermined position on the substrate. A fully automatic die bonder has both automatic die pick-up and placement. The die bonding machine used by the inventor is a fully automatic "pick and place" die bonder, but this invention is not limited to only this type of machine.
A typical bonding tool, or collet, for holding a LEDs die during positioning is shaped generally, on its holding face, with a concave opening for contacting a face of a die, a centrally positioned opening within the conical volume, and a vacuum source connected through the opening to the conical volume to produce a partial vacuum within the conical volume. The partial vacuum holds the die to the tool. The tool may then be positioned by known servo means, and the die released by opening the vacuum.
It is apparent that other positioning means may be used to position the die on a surface or substrate. The invention is useful where the die is smaller than the tool so that the dice cannot be positioned to touch each other.
Typical semiconductor LEDs are produced as a wafer matrix of LEDs which require lapping to reduce their height from an initially fabricated height to a specified finished product height. After lapping is completed, the wafer is saw-cut or diced into individual LED chips or dice. In the prior art matrices, the dice are desired to be all the same height. Wafers can be furnished and used at any height over a predetermined range of heights. The individual dice are separated from the wafer and accurately positioned as discrete elements in an array.
The maximum dice density, and hence resolution, attainable using existing manufacturing techniques is on the order of fifty lines per inch. The method herein described allows line resolution of at least one hundred lines per inch.